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Science: Neptune’s new moon baffles the astronomers

By JOHN MASON

THE discovery by the Voyager spaceprobe of a new Neptunian moon is causing
planetary scientists to rethink their theories about the origin of the planet’s
largest satellite, Triton. The new moon, 1989 N1, orbits Neptune every 1
day 3 hours in a nearly circular orbit at the planet’s equator. The orbit
is 92 700 kilometres above the planet’s cloud tops.

The orderly nature of its orbit contrasts sharply with the other two
known Neptunian moons, Triton and Nereid, which follow very unusual paths
around the planet. Triton’s orbit, which is also circular, is inclined at
about 20 degrees to the planet’s equator, but the moon moves in a retrograde,
or backwards, direction. It is the only large planetary moon to do so. Nereid
has a direct, or forwards motion, but its orbit is tilted at 30 degrees
to Neptune’s equator. It is the most elliptical of any known planetary satellite.

Before the recent discovery, scientists thought that because of Triton’s
peculiar retrograde and tilted orbit, it must have been a body wandering
the Solar System alone when it was captured by Neptune. Its initial path
around Neptune would have been non-circular, so the theory goes, but, over
a period of hundreds of millions of years, it became more circular as tidal
forces siphoned energy from its orbital motion. Now, with the discovery
of 1989 N1, this theory seems not to fit the available evidence.

‘The difficulty is that we have found a moon in a place we didn’t think
one should have existed,’ said Voyager’s assistant project scientist, Ellis
Miner of NASA’s Jet Propulsion Laboratory in Pasadena. ‘If Triton were a
relative newcomer to the Neptune system,’ he said, ‘it would have passed
near enough to the low orbit of any pre-existing moon, such as 1989 N1,
either to collide with it or sweep it up through gravitational attraction.’

The existence of 1989 N1 in the orbit it now occupies suggests that
Triton may not be a captured object after all. Instead, it may be a native
to Neptune. Indeed, the fact that Triton is so much larger than all other
known planetary satellites which have retrograde orbits had already led
some scientists to question the view that Triton is a captured object. Theoreticians
are now struggling to see if the ‘Triton-as-a-captured-object’ theory can
be salvaged or if alternative explanations for its unusual retrograde orbit
can be found. According to Archie Roy of Glasgow University, ‘The whole
of our handed down beliefs on the origin of Triton will have to be scrapped,
and we shall have to start again from scratch.’ John Mason